Conventional inkjet printers typically operate by ejecting small droplets of ink from individual orifices in an array of such orifices provided on a nozzle plate of a printhead. The printhead may form part of a print cartridge which can be moved relative to a sheet of paper and the timed ejection of droplets from particular orifices as the printhead and paper are relatively moved enables characters, images and other graphical material to be printed on the paper.
A typical conventional printhead is shown in FIGS. 1 to 5 which are, respectively, a simplified schematic plan view of a prior art inkjet printhead, a schematic cross-section of the printhead of FIG. 1 taken on the line II-II of FIG. 1, and a schematic cross-section of the printhead of FIG. 1 taken on the line III-III of FIG. 2. The printhead is fabricated on a silicon substrate 10 having thin film resistors deposited on its front surface, i.e. the surface facing the viewer in FIG. 1 and uppermost in FIGS. 2 and 3. The resistors are not visible in FIG. 1 due to the presence of an overlying nozzle plate 14 to be described. The resistors are arranged in an array relative to one or more ink supply slots (also not visible) in the substrate, and a layer of barrier material 12 is formed on the substrate around the resistors to isolate each resistor inside a respective thermal ejection chamber. The barrier layer 12 is shaped both to form the walls of the thermal ejection chambers and to provide an ink communication channel between each chamber and the ink supply slot. In this way, the thermal ejection chambers are filled by capillary action with ink from the ink supply slot, which itself is supplied with ink from an ink reservoir in the print cartridge of which the printhead ultimately forms part.
The composite assembly described above is typically capped by a nozzle plate 14, for example of nickel or polyimide. The nozzle plate has an array of orifices 16 which correspond to and overlie the ejection chambers so that each orifice is in register with a respective thin film resistor. The printhead is thus sealed by the nozzle plate 14, but permits ink flow from the print cartridge via the orifices 16 in the nozzle plate. In the illustrated example the orifices 16, and hence the underlying resistors, are disposed in three arrays, each array being in communication with a different coloured ink reservoir.
The thin film resistors are connected by thin film conductive traces to a plurality of electrical terminal pads 18 likewise deposited on the front surface of the substrate 10, typically in two rows arranged along opposite edges of the substrate 10 as seen in FIG. 1. These terminal pads 18 are exposed at the edges of the barrier layer 12 and, when the printhead is mounted on a print cartridge, they are connected via respective cantilevered flex beams 20, FIGS. 4 and 5, to corresponding traces on a flexible printed circuit 22 mounted on the print cartridge. Only the portion of the flexible printed circuit 22 in the immediate vicinity of the substrate 10 is shown in FIGS. 4 and 5. The terminal pads 18 are typically made of an aluminium-copper alloy and are gold plated to ensure a good electrical connection to the flex beams 20.
The flexible printed circuit 22 enables printer control circuitry located within the printer to selectively energise individual resistors under the control of software in known manner. When a resistor is energised it quickly heats up and superheats a small amount of the adjacent ink in the thermal ejection chamber. The superheated volume of ink expands due to explosive evaporation and this causes a droplet of ink above the expanding superheated ink to be ejected from the chamber via the associated orifice in the nozzle plate.
The two rows of terminal pads 18, together with their associated flex beams 20 and the adjacent edges of the nozzle plate 14 and printed circuit 22, are encapsulated in a UV cureable acrylic adhesive to avoid ink shorting due to ink ingress into the bond region. This cured adhesive forms ridges 24 above the level of the nozzle plate 14 and mandates a minimum distance d between the nozzle plate 14 and paper (indicated by the dashed line in FIG. 5).
The typical printhead described above is normally manufactured simultaneously with many similar such printheads on a large area silicon wafer which is only divided up into individual printhead dies at a late stage in the manufacture.
Many variations on this basic construction will be well known to the skilled person. For example, the number of arrays of orifices and ink ejection chambers provided on a given printhead may be varied, according to the number of differently coloured inks to be printed. The configurations of the ink supply slots, printed circuitry, barrier material and nozzle plate are open to many variations, as are the materials from which they are made and the manner of their manufacture.
The conventional printhead described above has certain disadvantages. The connections between the contact pads 18 and the flexible printed circuit 22 are usually made at the front surface of the substrate, as indicated in FIGS. 4 and 5. In practice this means that the print head has to be attached to the flexible circuit 22 before the flexible circuit is attached to the print cartridge body. Rework of this assembly is very difficult. Furthermore, the ridges 24 encroach on the nozzle plate to paper clearance, requiring their spacing to increase to reduce the risk of encapsulant-paper interference. This increase in spacing increases ink to paper trajectory errors, reducing print quality. In the case of printhead arrays, all interconnects take place on the nozzle side of the printhead, followed by encapsulation. It is likely that such printhead arrays would contain a percentage of defective printheads, and rework of an encapsulated array is very difficult.
U.S. Patent Application No. US2003/0082851 A1 describes a method of mounting a printhead die to a substrate by inserting upstanding conductive posts on the substrate through via holes in the die, and applying solder beads at the top ends of the posts. This is a complex and costly method which requires precise alignment of the posts with the via holes. Another technique described in the above US Patent Application uses via holes in the die whose internal walls are conductively plated. In this case a bevelled conductive cap is biased into the correspondingly bevelled upper end of the via hole, and a spring is biased into contact with the lower end of the via hole. This, too, is complex and costly technique.
It is therefore an object of the invention to provide an improved method of making an inkjet printhead in which, at least in certain embodiments, at least some of the disadvantages discussed above can be avoided or mitigated.